The present invention relates to methods of producing metal sandcastings, in which a set of castings is formed incorporating the casting runners and risers and is then subjected to a separating operation employing mechanical shocks to cause separating fractures at points predisposed to break which are situated at the margins of each casting at the points where the said risers, runners and other connections between the castings are located. Hereinafter such a method will be referred to as "of the kind described".
Methods of the kind described are generally employed with cast iron, and in particular white cast iron which has a structure of, or approximating to, the cementite type which exhibits extreme hardness. In many cases the castings have to be subjected to heat treatment to alter the structure of the cast iron into a structure of the ferritic or ferritic-pearlitic kind characteristic of ductile cast irons such as speroidal graphite cast iron.
To avoid this latter re-treatment operation, efforts have therefore been made to produce "as cast" ductile cast iron directly in the course of the casting operation. To achieve this object, it has been found necessary to control the operation very carefully from the metallurgical point of view by not only making provision for the introduction of additives but also by carefully controlling the rate of cooling.
Although the introduction of additives is an operation which can be managed perfectly well, provided that a great deal of care and experience are employed, the rate of cooling generally has to be sufficiently slow to avoid the formation of a cementite structure and in the majority of cases to obtain the highest possible ferrite content (in the case of cast iron of an average ultimate tensile strength, of the order of 38 kg/mm.sup.2), or a minimum pearlite content (in the case of cast iron of considerably higher tensile strength). To this end, it has been proposed to increase the amount of metal cast by providing large risers and by making the casting runners oversize, which slows down the general solidification by virtue of the thermal screening effect which the said risers have with respect to the thermal flux emitted by the casting as it cools. This procedure is not very economical however since it results in a high proportion of rejects, which have to be salvaged, that is to say physically handled and re-melted. This disadvantage is felt all the more severely when the size of the casting is small in relation to the size of the runners and risers since the heat loss remains considerable, whereas the natural mass available to form a thermal store is smaller. For this reason, and also to increase the efficiency with which the mold boxes are used, it has been proposed to arrange the mold cavities for such castings in pairs lying one on either side of the joint plane of the mold with at least one spacing core interposed, and to provide for multiple connecting zones between the two paired castings transversely to the joint plane of the mold, with recesses to form risers provided if required. This ensures a mutual thermal screening effect between the two castings situated facing one another, and also ensures that more intensive use is made of the volumetric capacity of the molding boxes. These advantages are sufficient to cause a preference for this latter method of molding in spite of the disadvantage represented by the need to position a spacing core between the castings, the presence of which is a necessary concomitant of this type of casting.
In comparison with conventional casting, the method of casting articles in pairs is also somewhat disadvantageous as regards the operation of breaking up a set of castings. In effect, in the case of a casting associated with a large riser, the riser comes between the casting runner and the casting and as a result the casting is suspended virtually alone at the end of the casting runner, which means that it is easy to fracture the casting runner close to the casting in the case of hard and brittle cast iron and does not create any major problems in the case of ductile cast iron straight from the mold i.e. "as cast". However, in the case of a set of castings cast in pairs with a large number of runners extending between the two castings in each pair at the points where the multiple connecting zones are situated, the separating operation, which is already laborious with extremely brittle white cast iron, becomes almost impossible with ductile cast iron straight from the mold and, even if it can be achievered with suitable tools, it results in a very substantial reject level as a result of castings breaking and other damage, which makes this method totally prohibitive with cast iron of this nature.
It should also be mentioned that a desperate measure which might have been thought of was cutting by oxygen torch, but is is precisely with "as cast" ductile cast iron that this process cannot be used since it destroys the structure in the neighbourhood of the cut. The only method remaining was apparently cutting by mechanical means, but this method is excessively slow and expensive, particularly in the case of pairs of castings with multiple connections which are very inaccessible to cutters.
It is for this reason that this method of casting has virtually been ignored hitherto despite the clear attractions which it has on a number of counts. It is an object of the invention to overcome or minimise this drawback. Other objects and advantages will become apparent from a study of the description which is to follow.